Annular centrifugal extractor with embedded stirring rotor

ABSTRACT

An annular centrifugal extractor with an immersed agitation rotor. The liquid-liquid centrifugal extractor including an emulsion chamber and a settling chamber, aligned because of the addition of a section of axis to the rotor, which allows flow and operation to be regularized, especially by allowing the dimensions of the chambers to be selected more freely.

The present invention relates to centrifugal extractor for an extractionprocess with two liquids.

Liquid-liquid extraction processes for constituents of mixtures viacentrifuge have currently been utilized for a long time in the chemical,pharmaceutical and nuclear industries, in particular. Certain equipmentutilized to carry out these processes are annular centrifugalextractors, in which the mixtures are introduced into a bowl of arotating rotor at one end and exit at the opposite end. The separationresults from decantation in which the heavy phase of the mixture iscentrifuged at the periphery of the bowl, while the light phase gatherscloser to the centre of rotation.

Extractors applying extraction to two liquids are generally doubleentry, that is, the liquid phases to be mixed and separated enter viagenerally different conduits. They are then mixed in an emulsion bymeans of the rotor before entering the bowl. One of these liquid phasesis called a supply phase and comprises solutes to be extracted, and theother is called a solvent and preferably harnesses certain of thesolutes. Mixing the phases followed by their separation has thesesolutes pass into the solvent and the extracts of the supply phase.Transfer of the solutes can also be carried out in the direction of thesolvent to the aqueous phase and is thus called disextraction.

Reference is now made to attached figures, in which:

FIG. 1 illustrates a known annular centrifugal extractor, and

FIG. 2 illustrates an extractor according to the present invention.

An example of known annular extractor is given in FIG. 1, where itessentially comprises a housing 1 and a rotor 2 turning inside it. Therotor 2 comprises a motor arbour 3, a bowl 4 and a block 5 connectingthese two to one another. It turns about a vertical axis, with the bowl4 being placed at the bottom. The housing 7 comprises two entry channels6 and 7 of the phases, which empty into an internal chamber 8 containingthe bowl, then flow into a narrow annular space 9 between the bowl 4 andthe housing 1, where they are emulsified by the chopping forces producedby rotation of the bowl 4. On arriving at the bottom of the housing 1,they flow under the bowl 4. Reliefs known as counter-blades 11 areinstalled at the base of the housing 1 to brake rotation of the emulsionand to direct it towards an orifice 10 located at the centre of thebottom of the bowl 4.

From the effect of the centrifugal force the emulsion from the phases isdecanted inside the bowl 4 after having passed over the orifice 10, theheavy phase gathering against the wall of the bowl 4 up to an interface12, and the light phase being beyond this interface 12 towards the axisof rotation. The separated mixture flows to the top of the bowl 4 andexits from there via a conduit 13 located to the side of the heavyphase, and through a first outlet 14 located to the side of the lightphase. The conduit 13 leads to an intermediate heavy phase chamber 15,which the heavy phase leaves by passing through a second outlet 15, andit discharges towards a gathering ring of the heavy phase 17. The lightphase passes into an intermediate chamber 19, then into an evacuationchannel 20, which leads it to a gathering ring of light phase 21.

The gathering rings 17 and 21 are located on the peripheral wall of thehousing 1, one above the other, while the above elements were worked inthe block 5 integral with the bowl 4. Outlet pipes 18 and 22 terminatein the gathering rings 17 and 21 conveying the separated phases to theexterior.

Certain refinements can complicate the structure of this annularextractor, but difficulties in correctly regulating the apparatus areencountered, in terms of the agitation creating the emulsion, thedecantation causing separation as well as the capacity to suction theemulsion into the bowl, depending above all on the rotation of the bowl4, and rotation speed reconciling optimal functioning of these threefunctions is often impossible to find. The most significant risk isexcessive aspiration of the phases in emulsion in the bowl, which wouldresult in a very current contact time and consequently to poor transferefficacy, or to aspiration of air. By reducing the speed of rotation ofthe rotor 3, this risk would be eliminated, but decantation would thenbe less.

It is also difficult to select favourable dimensions for the elements ofthe extractor, and in particular the bowl 4, of which the diameter andthe height must be sufficient to produce the chopping of emulsion, whichdevelops only with the narrowness of the interval between the housing 1and the bowl 4, and must also be exerted over a sufficient flow lengthin the space 9. It eventuates that the volume of the bowl 4 becomessignificant, and what is known as the hold-up (the stock taking) of theextractor increases, that is, the quantity of liquid held back there andwhich is to be reduced since it is temporarily subtracted in othertreatments in the industrial process, and that it complicates emptyingthe extractor during shutdown.

It is also a disadvantage that the rotation of the liquid in emulsionmust be braked by the counter-blades 11 to direct it to the orifice 10,before resuming rotation occurs in the bowl 4, causing energy loss.Finally, it has sometimes been stated that the interface 12 was notalways in the expected range, and this makes operation of the extractoruncertain.

All these factors explain certain operating limits, in particular forthe separation rate, encountered in this type of extractor.

In its most general form the invention consists of a centrifugalextractor of two fluid phases, comprising a central rotor turning aboutan axis of rotation and comprising a bowl, a stator, a settling chamberin the bowl of the rotor and an emulsion chamber between the rotor andthe stator, the emulsion and settling chambers communicating betweenthem and the emulsion chamber being placed under the settling chamber,characterised in that the emulsion and settling chambers are aligned insuccession on the axis of rotation of the rotor, the rotor comprising asection of axis delimiting the emulsion chamber.

The consequences of this original arrangement can be put as follows. Theradius dimensions of the emulsion and settling chambers becomeindependent, allowing them to be better selected. The emulsion chambercan thus be constructed with a small radius so that a Couette-Poiseuilleflow can be installed there, which is favourable to the regularity ofthe process, whereas the settling chamber can remain fairly wide with agreater radius than that of the emulsion chamber, which was excluded bythe preceding construction, but allows superior centrifugal forces to bemaintained.

The heights (or lengths) of the two chambers are also independent,allowing for a short settling chamber whereof the volume remainsmoderate, but a long and narrow emulsion chamber to benefit the latter,while the settling chamber was longer than the emulsion chamber in theknown design.

An important aspect of the invention is modular construction, whereinthe rotor and the stator are separably made up of assemblable portions,the rotor comprising a first portion having a peripheral wall of thesettling chamber and a second portion comprising the section of axis.Then it becomes possible to select the dimensions of the emulsion andsettling chambers and to adapt them to particular conditions of theextraction process, if sets of different pieces are used to make up therotor and the stator. According to particular arrangements, the secondportion of the rotor comprises a base of the bowl attached to the firstportion, and radii joining the base to the section of axis; or thestator comprises two superposed portions whereof one is hollowed by acavity delimiting an external wall of the emulsion chamber; or again,the bowl is a mobile discharge portion of a light phase outside thesettling chamber.

The movement of flow is that much more regular between the emulsionchamber and the settling chamber since their alignment is accompanied bypassage from one to the other, without switching rotation of the fluidusing counter-blades or means of the same type.

The narrow emulsion chamber discharges a significant volume at the baseof the extractor, which can be occupied by inlet channels of the phasesextending substantially vertically at the side of the emulsion chamberand joining it at their base. These channels serve as retainingchannels, that is, reservoirs partially filled from the phases beforethey pass into the emulsion chamber. It then becomes improbable toaspirate air in the emulsion chamber, which maintains the constantvolume placed in emulsion, and regularises the work of the extractor asmuch in emulsion as in settling.

Another beneficial arrangement is obtained if the section of axis of therotor has an increase in radius, on a portion extending into thesettling chamber, in the direction of the emulsion chamber.

Another possibility of air inlet, this time downstream of the emulsionchamber, is then blocked if the section of axis has a periphery plunginginto the liquid of the settling chamber at the site where it is thewidest.

A final refinement to be mentioned here is realised if intermediatechambers of the phases, located downstream of the settling chamber andlimited downstream by annular outlets, communicate with one another. Theresulting equalisation of pressure tends to reduce uncertainties on theposition of the interface.

All these refinements and others still will emerge from the commentaryto FIG. 2, which illustrates a preferred embodiment of the invention.The housing (stator) is here designated by reference 25, the rotor byreference 26, and its rotating axis by reference 27. The rotor 26 againcomprises a bowl 28, extending this time up above the housing 25, andalso a section of axis 29 coaxial to the axis 27 and extending under thebowl 28, almost as far as below the housing 25, and also inside the bowl28. The emulsion chamber 32 is delimited by the section of axis 29 andthe wall (thicker than in the known embodiment) of the housing 25; it isprolonged by a certain height inside the bowl 28 by an annular lip 69depending on the housing 25 and which contributes to delimiting theemulsion chamber 32 so as to prevent liquids from entering the spacelocated between the bowl 28 and the housing 25. Positioned in the lowerpart of said stator are blades 61 which send back any liquids present inthe abovementioned space, for example in the event the apparatus startsup in the presence of liquid. The emulsion chamber 32 is fed by inletchannels 30 and 31 of the two phases, which extend substantiallyvertically over the entire height of emulsion chamber 32 and join it viaa vertical connecting channel 34 and a lower orifice 33. The liquidphases fill the majority of the channels 30 and 31 under anycircumstances, and are set in rotation only after they have traversedthe orifice 33.

The top of the section of axis 29 is fitted with a mobile portion 35 inthe form of a muff, whereof the radius increases towards the base byforming a disc 38 restraining the section of the settling chamber 35,which extends about it. The liquid volume left to decant is controlledby the volume of the selected mobile portion 35, and by an annularoutlet 37, whereof the internal radius is less than that of this disc38, such that the latter is immersed and prevents air from entering theemulsion chamber 32 via downstream, just as the liquid filling thechannels 30 and 31 offered the same protection upstream, and the mobileportion 35 also carries blades 56 for conveying the liquid into thesettling chamber 36.

Extending beyond the outlet 37 is an intermediate chamber for the lightphase 39, which communicates with a gathering ring of the light phase 40which can be arranged in a helix, that is, its base is stepped down asfar as an outlet channel 41 to benefit the flow outside the separator. Ahollow channel 42 in the bowl allows the heavy phase to exit from thesettling chamber 35 and to join an intermediate chamber for heavy phase43 and to leave it via a second annular outlet 44. The heavy phase thenrejoins a gathering ring 45, which also can optionally be in a helix orstepped, then leaves the separator via an outlet channel 46 terminatingat the lowest point of the gathering ring 45. A communication 47connects the intermediate chambers 39 and 43 and equalises the pressurethere, thus reducing the parameters of uncertainty on the position ofthe interface 48 in the settling chamber 36. It will be noted that theannular outlets 37 and 44 are both mobile pieces, which allows them tobe easily replaced by others for changing the conditions for controllingthe separator. For this, the bowl 28 comprises a screwed base 57, which,when removed, allows removal of the section of axis 29 then the mobileportion 35 of the decanter which is fixed or assembled on the screwedbase 57, and joined thereto via radii 56. In addition to this, thehousing 25 comprises an upper portion 67 and a lower portion 68, whichare superposed on one another with centring adjustment and gaskets. Thelower portion 68 contains the channels 30 and 31, and the emulsionchamber 32 is hollowed out there; the upper portion 67 contains the bowl28.

This construction offers modularity of the extractor, which constitutesan essential advantage of the invention. The rotor 26 can be composed ofsection of axis 29 and bowls 28 selected from the respective sets ofdifferent dimensions, and similarly the housing 29 can be made up oflower 68 and upper 67 portions selected from the respective sets ofdifferent dimensions, above all for the cavities of the emulsion chamber32 and for the bowl 28.

The result of this is that dimensions of the emulsion chamber 32 and ofthe bowl 28 can be selected by the user in accordance with the processto be applied: the width of the emulsion chamber 32 will depend on thedesired intensity of the chopping creating the emulsion; the radius ofthe bowl 28 will depend on the desired intensity of the centrifugalforce creating separation of the phases; the height of the bowl 28 willdepend on the desired inventory volume. It will not be necessary to seekto modify the process by changing the rotation speed of the rotor 26.Another possibility for regulating the volume of the bowl 28, whichcontains the phases, consists of replacing the outlet 37 by another, ofwhich the lower edge has a different radius.

The bowl 28 thus comprises, apart from the screwed base 57, a main body50 containing the external wall of the settling chamber 36 and thusdetermines, solely with outlet 37, the volume of the phases retained inthe bowl 28; and finally, at the top, a screwed cover 58 (bearing theaxis 27) which allows withdrawal of the outlet 44, as soon as it hasbeen unscrewed. The entire rotor 26 can be withdrawn from the housing 25after the cover 59 has been removed therefrom.

A deflector 49 extends to the top of the body 50 belonging to the rotorand containing the intermediate chambers.

Finally, the extractor comprises a pair of windings 51 and 52 in thebase of the housing 25, both hollowed out by vertical evacuation ducts53 and 54 whereof the first can be butt-welded to a tapping channel 55leading underneath the bowl 28, in order to proceed with tapping mixedliquids, and whereof the second is butt-welded to the bass of thechannel 34 to empty the extractor.

The supply phase rate treated in a prototype was three times larger thanwith known apparatus (with a steady extraction rate of solutes) for thesame hold-up total.

1. A centrifugal extractor of two fluid phases, comprising: a centralrotor rotating about an axis of rotation and comprising a bowl, astator, a settling chamber in the bowl of the rotor, and an emulsionchamber between the rotor and the stator, the emulsion and settlingchambers communicating between them and the emulsion chamber beingplaced under the settling chamber, wherein the emulsion and settlingchambers are aligned in succession on the axis of rotation of the rotor,the rotor comprising a section of axis delimiting the emulsion chamber,and wherein the stator and the rotor are composed of separably assembledportions, the rotor comprising a first portion comprising a peripheralwall of the settling chamber and a second portion comprising the sectionof axis.
 2. The extractor as claimed in claim 1, wherein the secondportion of the rotor comprises a base of the bowl attached to the firstportion, and radii joining the base to the section of axis.
 3. Theextractor as claimed in claim 1, wherein the stator comprises first andsecond superposed portions with centering adjustment, the firstsuperposed portion being hollowed out of a cavity for the bowl and thesecond superposed portion being hollowed out from a cavity delimiting anexternal wall of the emulsion chamber.
 4. The extractor as claimed inclaim 1, wherein the settling chamber has a greater radius than theemulsion chamber.
 5. The extractor as claimed in claim 1, wherein thesection of axis of the rotor has a radius increase, on a portionextending into the settling chamber, going towards the emulsion chamber,the portion having a greater radius than an opening radius of a portionof the bowl, by which a light fluid phase discharges outside thesettling chamber.
 6. The extractor as claimed in claim 1, whereinintermediate chambers of phases, located downstream of the settlingchamber and limited by annular outlets, communicate with one another. 7.The extractor as claimed in claim 1, wherein the section of axis extendsinto the settling chamber and comprises blades.
 8. The extractor asclaimed in claim 1, further comprising gathering rings of phasesdownstream of the settling chamber and that have a base inclined towardsoutlet orifices of the separator.
 9. The extractor as claimed in claim1, wherein the bowl comprises a mobile discharge portion of a lightphase outside the settling chamber.
 10. The extractor as claimed inclaim 1, wherein the stator comprises an annular lip contributing todelimit the emulsion chamber and extending into the bowl.
 11. Acentrifugal extractor of two fluid phases, comprising: a central rotorrotating about an axis of rotation and comprising a bowl, a stator, asettling chamber in the bowl of the rotor, and an emulsion chamberbetween the rotor and the stator, the emulsion and settling chamberscommunicating between them and the emulsion chamber being placed underthe settling chamber, wherein the emulsion and settling chambers arealigned in succession on the axis of rotation of the rotor, the rotorcomprising a section of axis delimiting the emulsion chamber, andwherein the emulsion chamber communicates with inlet and retentionchannels of phases extending substantially vertically to a side of theemulsion chamber and leading by lower ends to a lower end of theemulsion chamber.